ECONOMY AND VARIETY OE COMrENSATIONS. 
Similar tubercles are introduced with the same 
■•idvantage of adding Strength as well as Beauty 
in many other cognate genera of chambered 
shells. (PI. 44, Fig. 9. 10. 14. 15.) 
In all these cases, we recognize the exercise of 
Hiscretion and Economy in the midst of Abun- 
dance ; distributing internal supports but spar- 
ingly, to parts which, from their external form, 
"'ere already strong, and dispensing them abuii' 
Fig. 7, a. c.) there is a double keel, produced by a deep depres- 
sion along the dorsal margin ; and the keels are fortified by a 
'ine of tubercles placed at the extremity of the transverse ribs. 
In the A. varians (PI. 37, 9. a. b. c.) which has a triple keel, 
the two external ones are fortified by tubercles, as iii Fig. 7, and 
the central keel is a simple convex arch. 
PI. 37, Fig. 8, oilers an example of domes, or bosses, com- 
pensating the weakness that, without them, would exist in the 
A. catena, from the minuteness of its ribs, and the flatness of 
the sides of the shell. These flat parts are all supported by an 
abundant distribution of the edges ot the transverse plates 
directly beneath them, whilst those parts which are elevated into 
bosses, being sufficiently strong, are but slightly provided with 
any other support. The back of this shell also, being nearly flat, 
(PI. 37, Fig. 8. b. c.) is strongly supported by ramifications of 
the transverse plates. 
In PI. 37, Fig. 6, which has a triple keel, (that in the centre 
passing over the siphuncle), this triple elevation affords compen- 
sation for the weakness that would otherwise arise from the great 
I>readtli and flatness of the dorsal portion of this species. Be- 
tween these three keels, or ridges, are two depressions or dorsad 
furrows, and as these furrows form the weakest portion of the 
**^'ell, a compensation is provided by conducting beneath them 
tile denticulated edges of the transverse plates, so that they pre- 
sent long lines of resistance to external pressure. 
